Trichomonas vaginalis P16 Immunoreactivity in cervicovaginal Pap tests: a diagnostic pitfall

Detection of p16 is emerging as a useful biomarker for Human Papillomavirus (HPV)-related dysplastic and malignant lesions of the cervix, and as such has potential application to cervicovaginal Papanicolaou (Pap) specimens. While evaluating p16 immunocytochemistry in our laboratory we observed one Pap test in which Trichomonas vaginalis stained positively for p16. We therefore proceeded to determine the frequency of T. vaginalis immunoreactivity for p16 in 10 consecutive, satisfactory, liquid-based Pap tests diagnosed as negative for intraepithelial lesion or malignancy in which T. vaginalis was present. For each case, a ThinPrep slide prepared from residual vial material was immunostained with p16. In an additional case, a prepared cell block was stained with p16. T. vaginalis were consistently p16 positive. We document, for the first time, p16 immunoreactivity of T. vaginalis that, on a Pap slide and cell block, may be morphologically misinterpreted as small dysplastic or malignant epithelial cells. The presence of this parasite in Pap tests may also potentially hinder the use of p16 as an adjunct to liquid-based cervical screening cytology.